Image data decompression

Abstract

There is provided a computer-implemented method and a decompression unit for decompressing a compressed block of data in accordance with a multi-level difference table. The compressed block of data represents a block of image data comprising a plurality of image element values, wherein each level of the multi-level difference table comprises a plurality of entries. An origin value for the block of image data is determined using data representing the origin value from the compressed block of data. A level within the multi-level difference table for the block of image data is identified using an indication of the level from the compressed block of data. For each image element value in the block of image data, a respective entry indication from the compressed block of data is used to identify one of the entries at the identified level within the multi-level difference table, and the image element value is determined using: (i) the determined origin value for the block of image data, and (ii) the identified entry at the identified level within the multi-level difference table for the image element value.

What is claimed is: 
     
       1. A computer-implemented method of decompressing a plurality of compressed blocks of data in accordance with a multi-level difference table, wherein the compressed blocks of data represent blocks of multi-channel image data and relate to respective channels of the multi-channel image data, wherein each block of image data comprises a plurality of image element values, and wherein each level of the multi-level difference table comprises a plurality of entries, the method comprising decompressing the block of image data for each of the different channels of the multi-channel image data independently by:
 determining an origin value for the block of image data using data representing the origin value from the compressed block of data; 
 identifying a level within the multi-level difference table for the block of image data using an indication of the level from the compressed block of data; and 
 for each image element value in the block of image data:
 using a respective entry indication from the compressed block of data to identify one of the entries at the identified level within the multi-level difference table; and 
 determining the image element value using: (i) the determined origin value for the block of image data, and (ii) the identified entry at the identified level within the multi-level difference table for the image element value. 
 
 
     
     
       2. The method of  claim 1 , wherein a size of a range of values represented by the entries in a level is different for different levels of the multi-level difference table. 
     
     
       3. The method of  claim 1 , wherein said determining the image element value comprises summing the determined origin value for the block of image data and a value represented by the identified entry at the identified level within the multi-level difference table for the image element value. 
     
     
       4. The method of  claim 3 , further comprising, if the result of the sum is greater than a maximum possible image element value, setting the image element value to be the maximum possible image element value. 
     
     
       5. The method of  claim 1 , wherein the data representing the determined origin value in the compressed block of data has fewer bits than any one of the determined image element values in the block of image data. 
     
     
       6. The method of  claim 1 , wherein the first entry in each level of the multi-level difference table represents a value of zero. 
     
     
       7. The method of  claim 1 , wherein the image element values each have 8 bits, wherein the block of image data comprises four image element values, such that the block of image data has 32 bits, and wherein the compressed block of data has 16 bits, comprising: (i) 5 bits of data representing the determined origin value, (ii) 3 bits for the indication of the level, and (iii) 2 bits for the respective entry indication for each of the four image element values. 
     
     
       8. The method of  claim 1 , wherein the compressed block of data is decompressed by performing at least one operation including an addition operation or a subtraction operation, but the method of decompressing the compressed block of data does not involve performing any multiplication operations or division operations. 
     
     
       9. The method of  claim 8 , wherein the image element values are in an integer format, and wherein said at least one operation operates on image element values in the integer format. 
     
     
       10. The method of  claim 1 , wherein the block of multi-channel image data is colour data for a block of colour values, comprising: (i) a first block of image data relating to a red channel of the block of colour values, (ii) a second block of image data relating to a green channel of the block of colour values, and (iii) a third block of image data relating to a blue channel of the block of colour values, and wherein after compressed blocks of data relating to the three blocks of image data are decompressed independently, colour recorrelation is performed on the colour values. 
     
     
       11. The method of  claim 10 , wherein the multi-channel image data is colour data in an RGB format such that each colour value comprises a red value (R), a green value (G) and a blue value (B),
 wherein said colour recorrelation comprises calculating the red values (R) using the image element values (R′) which have been decompressed from the first compressed block of data and using the image element values (G′) which have been decompressed from the second compressed block of data in accordance with the equation R=R′+G′, 
 wherein said colour recorrelation comprises determining the green values (G) using the image element values (G′) which have been decompressed from the second compressed block of data in accordance with the equation G=G′, and 
 wherein said colour recorrelation comprises calculating the blue values (B) using the image element values (B′) which have been decompressed from the third compressed block of data and using the image element values (G′) which have been decompressed from the second compressed block of data in accordance with the equation B=B′+G′. 
 
     
     
       12. The method of  claim 1 , wherein three blocks of compressed data are decompressed independently to determine multi-channel colour data in a luma-chroma format comprising: (i) a first block of image data relating to a luma channel (Y), (ii) a second block of image data relating to a first chroma channel (Cb), and (iii) a third block of image data relating to a second chroma channel (Cr), and wherein the method comprises converting the colour data into an RGB format such that each colour value comprises a red value (R), a green value (G) and a blue value (B). 
     
     
       13. A method of decompressing a compressed block of data, comprising:
 reading an indication in a header associated with the compressed block of data which indicates whether the compressed block of data has been compressed with a lossless compression technique or with a lossy compression technique; and 
 if the indication indicates that the compressed block of data has been compressed with the lossless compression technique, performing a lossless decompression technique on the compressed block of data to thereby decompress the compressed block of data; and 
 if the indication indicates that the compressed block of data has been compressed with the lossy compression technique, performing the method as set forth in  claim 1  to thereby decompress the compressed block of data. 
 
     
     
       14. The method of  claim 1 , further comprising outputting the block of image data for further processing. 
     
     
       15. A decompression unit configured to decompress a plurality of compressed blocks of data in accordance with a multi-level difference table, wherein the compressed blocks of data represent blocks of multi-channel image data and relate to respective channels of the multi-channel image data, wherein each block of image data comprises a plurality of image element values, and wherein each level of the multi-level difference table comprises a plurality of entries, the decompression unit being configured to decompress the block of image data for each of the different channels of the multi-channel image data independently by:
 origin value determination logic determining an origin value for the block of image data using data representing the origin value from the compressed block of data; 
 level identification logic identifying a level within the multi-level difference table for the block of image data using an indication of the level from the compressed block of data; and 
 image element value determination logic, for each image element value in the block of image data:
 using a respective entry indication from the compressed block of data to identify one of the entries at the identified level within the multi-level difference table; and 
 determining the image element value using: (i) the determined origin value for the block of image data, and (ii) the identified entry at the identified level within the multi-level difference table for the image element value. 
 
 
     
     
       16. The decompression unit of  claim 15 , wherein the image element value determination logic is configured to determine an image element value of the block of image data by summing the determined origin value for the block of image data and a value represented by the identified entry at the identified level within the multi-level difference table for the image element value. 
     
     
       17. The decompression unit of  claim 15 , wherein the origin value determination logic, the level identification logic and the image element value determination logic are configured in dedicated hardware. 
     
     
       18. A decompression unit configured to decompress a compressed block of image data, the decompression unit comprising:
 a first decompression unit configured to perform lossless decompression on a compressed block of image data; 
 a second decompression unit as set forth in  claim 16 , configured to perform lossy decompression on a block of image data; and 
 decompression technique determination logic configured to read an indication in a header associated with the compressed block of data which indicates whether the compressed block of data has been compressed with a lossless compression technique or with a lossy compression technique, 
 wherein the decompression technique determination logic is configured to, if the indication indicates that the compressed block of data has been compressed with the lossless compression technique, cause the first decompression unit to perform lossless decompression on the compressed block of data to thereby decompress the compressed block of data, and 
 wherein the decompression technique determination logic is configured to, if the indication indicates that the compressed block of data has been compressed with the lossy compression technique, cause the second decompression unit to perform lossy decompression on the compressed block of data to thereby decompress the compressed block of data. 
 
     
     
       19. A non-transitory computer readable storage medium having stored thereon a computer readable description of a decompression unit that, when processed in an integrated circuit manufacturing system, causes the integrated circuit manufacturing system to manufacture an integrated circuit embodying the decompression unit, wherein the decompression unit is configured to decompress a plurality of compressed blocks of data in accordance with a multi-level difference table, wherein the compressed blocks of data represent blocks of multi-channel image data and relate to respective channels of the multi-channel image data, wherein each block of image data comprises a plurality of image element values, and wherein each level of the multi-level difference table comprises a plurality of entries, the decompression unit being configured to decompress the block of image data for each of the different channels of the multi-channel image data independently by:
 origin value determination logic determining an origin value for the block of image data using data representing the origin value from the compressed block of data; 
 level identification logic identifying a level within the multi-level difference table for the block of image data using an indication of the level from the compressed block of data; and 
 image element value determination logic, for each image element value in the block of image data:
 using a respective entry indication from the compressed block of data to identify one of the entries at the identified level within the multi-level difference table; and 
 determining the image element value using: (i) the determined origin value for the block of image data, and (ii) the identified entry at the identified level within the multi-level difference table for the image element value.